Compilare: main.cpp: In function 'bool pair_up(int)': main.cpp:31:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < a[x].size(); i++) { ~~^~~~~~~~~~~~~ main.cpp:39:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i = 0; i < a[x].size(); i++) { ~~^~~~~~~~~~~~~ main.cpp: In function 'void solve()': main.cpp:53:12: warning: unused variable 'x' [-Wunused-variable] int i, x, y; ^ main.cpp:53:15: warning: unused variable 'y' [-Wunused-variable] int i, x, y; ^